The aquifers being exploited are mostly the unconsolidated Quaternary alluvium and the Cuddalore sandstones of the Tertiary sequence overlying the Cretaceous.

CGWB studies reveal that groundwater in the area occurs under unconfined, semi-confined, as well as confined conditions.

The two aquifers representing the unconfined and semi-confined conditions occur within 100 m depth. They are underlain by a 10 to 15 m thick lignite bed.

The other two aquifers at depth underlying the lignite bed are under confined conditions.

The aquifers occurring above as well as below the lignite are quite thick and separated from each other by 10-15 m thick clay beds.

In the coastal part, fresh water aquifers are overlain by a thick saline water zone in the Quaternary formations.

The unconfined aquifer is 2 to 30 m thick. It consists of laterite, lateralized sandstone, clayey sandstone, mottled ferruginous sandstone (Cuddalore formation-Tertiary), and alluvium (Recent). The aquifer material is fined to medium grained. Shallow aquifers are developed for domestic and irrigation purposes through tubewells and dugwells. The groundwater level in some areas has gone down to 20 m below ground level, resulting in drying up of dugwells.

The semi-confined aquifer occurs at a depth of 40 – 100 m and overlays the lignite bed. It comprises clayey sandstone, sand, sand with pebble and gravel. The aquifer is developed by tubewells for irrigation purposes. The piezometric surface in the aquifer is of the order of 20 to 50 m below ground level.

The confined aquifers occur below the lignite bed and can be classified as upper confined (100-140m bgl) and lower confined (below 140m bgl) aquifers. A thin discontinuous clay layer separates the lower from the upper confined aquifer. The aquifer material comprises very fine to coarse sand with occasional clay intercalations. The aquifers are developed by deep tubewells for irrigation purposes. The piezometric head varies from 60 to 105 m bgl. Two to three decades ago, there were free flowing conditions observed during exploration of these aquifers. Nowadays, they have generally become sub artesian.

The lower and upper confined aquifers discussed above have a discharge of over 10 to 65 litres per second. The transmissivity and storativity of these aquifers ranges from 150 to 4000 m2/day and 7.72 to 9.58*10^5 respectively.

Water level

The pre-monsoon (May 2011) depth to water level contour map for the top unconfined aquifers shows that the groundwater level is less than 10m bgl in most of the area, except for the north western part where it ranges from 15 to 20m bgl.

The pre-monsoon (May 2011) depth to piezometric surface in the wells tapping the deeper aquifers (more than one zones) shows that the piezometric surface ranges from 20 to 60 m bgl in most of the area, except for a small pocket in the northern part of the area where it is more than 60 m bgl. In general it is observed that the piezometric surface is deep in the north western part, and becomes shallow towards the coast.

Geophysical studies

CGWB, GSI and CSRI-NGRI have conducted electrical resistivity soundings (VES) in the area in the past. These VES reveal the presence of fresh aquifers at depth in the coastal part, underlying the thick saline water zone in the Quaternary formations.

Exploration

The boreholes already drilled by CGWB tapped the aquifers to a maximum depth of 471 m, providing information on the regional disposition of the aquifers. Within this depth range, 4 aquifer groups are identified: unconfined, semi-confined, upper confined, and lower confined aquifers. The hydraulic conductivity of the aquifers ranges from 7 to 143 m/day.

In the central western part of the watershed, the hydrogeological cross-section based on CGWB wells reveals the disposition of Cuddalore sandstone aquifers. The aquifers become thick towards the south.

Figure 6.4: Geological cross-section (1).

The north-south cross-section in the coastal tract of the area is shown below, revealing the fresh aquifer in the Cuddalore sandstone, up to a depth of 200 m bgl.

Figure 6.5: Geological cross-section (2).

Groundwater quality

In general, the quality of groundwater is good in the project area, except for groundwater salinity in the eastern most coastal part.

Groundwater resources

The stage of ground water development in the phreatic / shallow aquifer is 71% and the area falls in the semi-critical category.